This invention relates in general to echo cancellation, and in particular to the suppression of echo leak through in the output of an echo canceller by improved near end speech pause determination.
Echo cancellers are commonly used in full duplex telephony systems to remove undesirable echo signals that would otherwise be included in an output signal. The undesirable echo signal is a far end audio signal that has been modified by a system that introduces either acoustic echo or electric echo and other undesirable inputs, such as background noise. Such telephony systems include telephone handsets operating in an acoustic environment of speaker and microphone coupling, telephone 4 to 2 wire and 2 to 4 wire hybrid audio signal converters, and such systems are generically referred to as echo generating systems. The far end signal, or the echo-generating signal, is alternatively called a reference signal 105.
Referring to FIG. 1, a functional block diagram of a telephony system 100 that incorporates a conventional echo canceller 150 is shown. The echo canceller 150 includes a synthetic echo generator 125 that has a linear adaptive filter that attempts to mimic the echo generating system 110 that is causing the echoing of the reference signal 105. The reference signal 105 is modified by the echo generating system 110, resulting in an undesirable (far end with echo) signal. A near end signal 115 (for example a speech signal from a telephone handset microphone) is added to the undesirable signal, resulting in a near-end-plus-far end echo signal 120 that is also known to those of ordinary skill in the art as the desired signal 120. The reference signal 105 is also coupled to an input of the echo canceller 150 that is coupled to the synthetic echo generator 125. A signal called a synthetic echo signal 126, that is an approximation of the undesirable (echo) signal, is generated from the far end signal by the synthetic echo generator 125 via adaptive filtering. This synthetic echo signal 126 is subtracted by a difference function 127 of the echo canceller 150 from the desired signal 120, generating an echo cancelled version of the desired signal, also known in the art as an error signal 135 (because during times when there is no near end speech, this signal should be at minimum levels). Since this linear operation is not completely effective, for several reasons, residual echo components remain in the echo cancelled error signal 135. One significant reason is that the echo generating system 110 may be non-linear.
An approach to improving the performance of echo cancellation, especially when echo generating consists of a large non-linear component, is to reduce the residual echo components during times when there is no speech input at the near end, because the residual components occurring during these times, called echo leak-throughs, are very noticeable. So, some conventional echo cancellation techniques make a determination of when a pause (or longer cessation) in near end speech occurs, and perform some type of non-linear operation by switching the output so that the output, called herein the leak through output, is either greatly reduced or the output is replaced entirely by internally generated noise known in the art as comfort noise. Comfort noise is used to reduce echo because complete silence on the output is uncomfortable to listeners at the far end.
But because of the non-linear nature of the echo generation system, echo cancellers will sometimes make an incorrect decision as to whether there is a valid pause in the near-end signal. With such an incorrect decision, either a valid echo cancelled near-end signal is mistakenly replaced by comfort noise or the echo-cancelled signal is passed through during a pause in near end audio. Either event results in undesirable signals occurring in the leak-through suppressed signal. This negative aspect of echo cancellation can still be annoying to a far end listener.
In the technique of switching to comfort noise during pauses, the synthetic echo generator 125 makes a determination from the error signal 135 as to when there is a pause or a longer cessation of the reference signal 105, and a control signal 131, is generated and used to select the output 171 of the echo canceller 150 (which is also the output of the telephony system 100) as being either the error signal 135 or an output of a comfort noise source 140 by means of a switch function 170. The control signal 131 is commonly used in conventional echo cancellers and is described herein as a center clipper signal. The center clipper signal 131 is typically generated based on an echo return loss (ERL) parameter derived from the linear adaptive filter, and although the use of the center clipper signal 131 does improve the performance of echo cancellation, in conventional echo cancellers 135 the use of the center clipper signal 131 still allows noticeable echo leak throughs because of non-linear echo generating systems 110, so the output 171 of the echo canceller 150 is called the leak-through output. The technique of attenuating the echo leak through during pauses is described in U.S. Pat. No. 5,894,512, issued to Nasu on Apr. 13, 1999. Nasu""s technique operates to attenuate what he calls peripheral noise, which can be echo and/or background noise when the power of the near end signal is less than the power of the reference signal, by attenuating the output signal.
These techniques, while quite effective in many situations, still leave some echo leak through as described above, so what is needed is a more effective echo leak through reduction technique that reduces echo leak through more accurately and completely than available techniques.